Testing Times For Interferometers

Interferometry is a long-established method for the testing of optical components and systems. By measuring the distortions in a wavefront interacting with a test piece compared to a reference beam, aberrations in optical systems, badly manufactured optical components and inhomogeneities in materials can be identified. The interference between the test beam and the reference beam appears as a set of light and dark fringes which yield information related to surface form errors or optical waveform distortion errors. The use of PC-based software to automate the fringe analysis and measurement processes has catapulted interferometry into the realms of "routine analysis" instrumentation, where users with little knowledge of optics can be expected to carry out interferometric investigations. However, good practice in measurement methodology should not be jettisoned simply because an instrument is software controlled. Interferometric measurements can vary from comparative tests applying visual assessment - generally used for relatively low accuracy results - to a calculation of the complete geometrical properties of an optical system. Commonly required measurements, however, are those of form error both for flat and spherical optics. Since all measurements are based on analysis of the fringe patterns, attention to detail in their generation and stability is of paramount importance.


Good Practice

The quality of results obtained depends on the quality of the interferometer and the reference optics used. A high quality reference optic is essential as a beam splitter and may be specified as
l/20 or better peak to valley for transmission flats and l/10 or better for transmission spheres, where l = 633 nm. For long wavelength interferometers, the references may be significantly better than this. Most software allows the subtraction of errors due to the reference components from the measured wavefront which relaxes the requirements for component accuracy somewhat. However, it is never completely certain that the same part of a reference component is being used, particularly if a wedged or decentred optical component has been inserted for measurement.

For the best results, all optical components should be mounted on a vibration-isolated optical bench. Whilst this is less critical for low accuracy comparative tests, it is absolutely essential for high accuracy phase shift analysis during which individual snapshots can be affected by vibration. During a statistical run of measurements, it is possible to see the effects on the results by the operation of plant machinery nearby, or the cutting-in of air conditioners. Air turbulence which could arise from air conditioners or forced convection in an open room, or the proximity of heat generating electronic equipment cannot be eliminated by anti-vibration systems. If critical measurements including long path lengths are to be made, it is often necessary to shield the equipment from sources of air convection.

The test beam and reference beam must follow an identical path inside the interferometer and the test beam must retrace its path accurately. Internal instrument alignment procedures as stipulated by the manufacturer must be followed and external optical components including the test piece and reference optics must also be carefully aligned along the optical axis of the interferometer. Accurate focusing of the optical pupil is also important, particularly at wavelengths in the infra-red, to avoid erroneous interpretation of results. Diffracted light will also interfere with the reference beam so the optical aperture of the system under test should be focused onto the camera sensitive surface. Diffraction effects arising from double pass configurations may be cured by imaging the optical pupil onto the interferometer camera both directly, and via the reference mirror or sphere. If the system under test has a larger physical diameter than the used aperture then it may be measured over the larger aperture and then masked in software to the used aperture to keep the effects of diffraction away from the pupil zone analysed.

When performing a measurement, as much of the interferometer's pupil as possible should be used, to maximise the number of data points in the measurement data. An optical zoom system or other aperture adapter is essential to provide such pupil matching. Electronic zoom simply enlarges both the image and pixels without giving any more data. It is also essential that the correct area of the component under test is measured. Optical components are usually defined to have a clear aperture over which the wavefront is controlled. If the component is significantly larger than the clear aperture, then extraneous zones should be ignored by masking physically or in software.


Fringe Analysis

Following optimisation, the fringes must be analysed. Traditional analysis is performed by simply printing the interferogram and measuring deviations with a ruler. More recently the image is captured digitally using a framegrabber and software used to analyse the fringes. This is known as static fringe analysis. Typically this offers an accuracy of around l/20 and has the benefit that the software results can be verified manually for simple peak to valley measurements. Software offers far more sophisticated analysis including calculation of the rms wavefront deviation, and other derived functions such as the Strehl Ratio, MTF, Point Spread Function and Encircled Energy Function. To get the most out of this software, users will almost certainly have to intervene to manually shift measurement points which have been misplaced. This may be due to changes of contrast in the interferogram, or points placed midway between a fringe and the edge of the pupil. Static fringe analysis does not provide many data points within the test pupil so that data at other points must always be interpolated or extrapolated. The wavefront shape is often approximated by fitting Zernike coefficients to the available data. This is normally fine for interpolated zones of the test pupil but for large order coefficient fitting, the extrapolated data can vary widely from the actual wavefront. Ideally, measurements should always be made over a larger area than required and the aperture reduced in software. Other extrapolation methods such as Spline fitting or simply linear extrapolation will give different results especially for peak to valley measurements.

Phase shifting techniques can improve accuracy by capturing the fringes with several different phases of the reference beam. This permits calculation of absolute phase for every pixel within the pupil which increases accuracy and repeatability of measurements to around l/100, and allows the sense of the fringe perturbation to be identified as a wavefront retardation or advance. Because the multiple measurements are made at different times, the measurement is more susceptible to vibration than static fringe analysis.

The above techniques may provide repeatable, but not necessarily correct results. Systematic errors may bias the results in one direction. These may be due to misalignment, distortion due to gravity, or distortion due to the mounting technique employed for components. Flimsy components especially should be mounted mimicking their final application mount and orientation. A vertically mounted interferometer may be required to mount components without asymmetric stresses.


International Standards

The basic methodology for making measurements is extremely important in spite of the introduction of sophisticated software. In addition, since different manufacturers' software use different algorithms for their calculations and extrapolations, there is a clear need for an international standard to avoid variations in actual performance levels compared to quoted capability. With such a standard, both methodology and software could be evaluated prior to "real" measurements. The relevant technical committee of the International Standards Organisation (ISO) has already initiated the writing of a draft standard on this topic and Precision-Optical Engineering recently participated with other manufacturers and laboratories in a "round robin" evaluation1 of interferometers and laboratories, organised by the corresponding committee of the British Standards Institution (BSI) to survey existing practice.

It is hoped that the introduction of such a standard will be welcomed throughout the industry.


Figure 1. Static fringe analysis output from P-OE's INTERFIRE 633 interferometer  showing digitisation of fringes and 2-D and 3-D representations of the Optical Path Difference of the measured sample.



Figure 2. Results from the INTERFIRE 633 interferometer equipped with P-OE's new phase shift analysis system.



Figure 3. Variation of measurement with static fringe analysis over 600 measurements.


Figure 4. VLS II integrated vertical looking interferometric workstation for vertical testing of components.

Reference1Interferometric optical testing: an interlaboratory comparison, J. D. Briers, Journal of Optics A, Pure & Applied Optics, 1, 1, 1999.